Processes involving
the contact of gasses and liquids with a solid wall are ripe for
discovery in the reduced gravity environment of orbit. Such processes,
although commonly experienced in everyday life, can quickly become
complicated once the details are scrutinized, although it is often
the understanding of the details that leads to new insights.

One such process
we wanted to observe was boiling; however, we could not figure out
how to do it safely until last week. We were setting up to make
some test solder joints using our soldering iron inside a maintenance
glovebox designed especially for such operations. The idea struck
me, why not use the soldering iron to boil water in thin film geometry
while inside the glovebox? By keeping the volume of water small,
perhaps a few milliliters, this operation could safely be done with
no fear of errant droplets of hot water causing damage.

A video camera
was set up with an old tee shirt for a backdrop. It is important
to record the observations with a background that enhances the view
of the bubbles. A standard 50-millimeter diameter water film was
made about 300 micrometers thick. A new solder tip was placed on
the soldering iron. We did not want rosin residues from prior soldering
to coat the water surface and change the surface tension dynamics.

Having set
all this up, we proceeded to slowly place the hot tip into the film.
It popped. Again and again it was repeated with the same results.
The hot tip would pop the film as soon as it made contact. We made
thicker films and finally had success when the film was 2 to 4 millimeters
thick. Three millimeters seemed like a good compromise with all
subsequent work done at this thickness.

When the tip
was inserted into the film, intense Marangoni convection was seen.
A trail of small bubbles produced by the hot tip acted as tracer
particles and made the convective flow visible. This Marangoni convection
was significantly more violent than the mild eddies that were driven
by my flashlight some weeks ago. It looked like the invisible spoon
of Marangoni had been exchanged for an eggbeater.

The soldering
iron tip did not seem to transfer sufficient heat to make the water
boil. Small bubbles formed locally at the tip, however, a true boil
was not produced. I was also dissatisfied with the geometry.

The tip was
cone-shaped with two small flats creating a screwdriver-type blade.
A rounded lip transitioned from the solder iron's conical tip to
its cylindrical-shaped barrel. The water film wanted to crawl up
the tip and hang at the transition zone. This created a rather complicated
geometry for the water-wall contact zone. When you design an experiment,
it is best to create a geometry that is simple and readily tractable
with mathematics. Nature is complicated enough as it is, and it
does not help matters to inflict more complexities into the analysis.

Since we had
only a soldering iron for a heat source, it was decided to penetrate
the film well past the tip so the cylindrical barrel was perpendicular
to the film. Now we had a nice simple geometry consisting of a heated
cylindrical wall, 8.5 millimeters in diameter, concentric and perpendicular
to a film of about 3 millimeters thick. Not only did this create
a simpler geometry, but it also created a greater contact area for
heat transfer.

When the barrel
was placed into the film, surface tension forces caused the water
to crawl up and down the barrel thickening the heated zone to perhaps
5 millimeters next to the wall, so the flat nature of the film was
somewhat distorted. Within minutes, there was a full rolling or
nucleate boil in the film. We were observing another "wow"
moment.

Small gas bubbles
formed at the wall and grew to perhaps 1 to 3 millimeters in diameter
before they were whisked away by convection. Once surrounded by
cooler water, the bubbles shrank to less than a millimeter and moved
in circular convection patterns that often brought them back by
the wall, although they would not re-attach. They aided in knocking
loose newly formed bubbles by colliding with them.

The convection
appeared to be driven by two components. Marangoni convection began
soon after heating and created well-developed, fixed convection
patterns just as small bubbles of less than a millimeter were beginning
to form. Some of the small bubbles stayed attached to the wall while
others popped off and "convected" away. Once the nucleate
boil formed, the expansion of the bubbles at the wall created a
mass flow away from the surface and added to the Marangoni convection.
This resulted in bubbles forming at the wall, growing to a few millimeters,
and then popping off and moving away.

It appeared
like the convective flow generated from the bubble expansion dominated
over Marangoni convection. To the first order, it looked like boiling
seen on the bottom of any kitchen pot in the process of making dinner.
I was both amazed and perhaps a little disappointed that boiling
in the absence of significant gravity was not more exotic. I was
hoping for at least an oscillating system where the water in contact
with the hot wall would be pushed away by an expanding gas film
until it burst and allowed water to once again contact the hot wall.
The universe obviously did not first consult me on how this should
work.

These observations
did incite some new thoughts. Nucleate boiling under natural convection
is a well-studied industrial process, and as classically developed
on Earth, has gravity-driven buoyancy terms in the equations used
for calculating heat transfer coefficients. Gravity supplies the
force needed to drive the bubbles away from the heated surface so
more water can move in, thus creating a steady state boiling process.
At the small scale of bubbles forming on a hot surface, maybe there
are other forces present as witnessed here on orbit, that are initially
more significant than gravitational forces. Perhaps these same forces
are what cause bubbles to pop off the surface and then move outward
where the grip of gravity-driven buoyancy takes over. However, maybe
Earth's gravity is the dominating force all along, and the observations
made here are simply caused by small tertiary forces kept at bay
by gravity and only allowed to exercise some authority in its absence.

Such questions,
spawned from the delights of dancing on the edge of the unknown,
take one right back to where you first started, except this time,
with a wiser smile.